1. Field of the Invention
The present invention relates to a plasmon generator for use in thermally-assisted magnetic recording where a recording medium is irradiated with near-field light to lower the coercivity of the recording medium for data writing, and to a thermally-assisted magnetic recording head including the plasmon generator.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) for reading and a write head including an induction-type electromagnetic transducer for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
To increase the recording density of a magnetic recording device, it is effective to make the magnetic fine particles of the recording medium smaller. Making the magnetic fine particles smaller, however, causes the problem that the magnetic fine particles drop in the thermal stability of magnetization. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technology so-called thermally-assisted magnetic recording. The technology uses a recording medium having high coercivity. When writing data, a write magnetic field and heat are simultaneously applied to the area of the recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. The area where data is written subsequently falls in temperature and rises in coercivity to increase the thermal stability of magnetization. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the recording medium. A known method for generating near-field light is to use a plasmon generator, which is a piece of metal that generates near-field light from plasmons excited by irradiation with laser light. The laser light to be used for generating the near-field light is typically guided through a waveguide, which is provided in the slider, to the plasmon generator disposed near a medium facing surface of the slider.
U.S. Pat. No. 7,330,404 discloses a technology in which the surface of the core of a waveguide and the surface of a plasmon generator are arranged to face each other with a gap therebetween, and evanescent light that occurs at the surface of the core based on the light propagating through the core is used to excite surface plasmons on the plasmon generator. Based on the excited surface plasmons, near-field light is produced.
The plasmon generator has a front end face located in the medium facing surface. The front end face includes a near-field light generating part which generates near-field light. The surface plasmons excited on the plasmon generator propagate along the surface of the plasmon generator to reach the near-field light generating part. As a result, the surface plasmons concentrate at the near-field light generating part, and the near-field light generating part generates near-field light based on the surface plasmons.
When a recording medium is irradiated with near-field light, an area having a higher temperature than its surroundings (this area will hereinafter be referred to as a heated spot) is formed in the recording medium. To increase the recording density, it is necessary to reduce the diameter of the heated spot.
A conventional plasmon generator is formed of a single material. The material is typically a metal having a high electrical conductivity, such as Ag, Au, Al, or Cu. However, the plasmon generator formed of a single material has the following three problems.
A first problem will be described first. The first problem relates to corrosion. More specifically, the front end face of the plasmon generator can be in contact with a high-temperature and high-humidity atmosphere. Therefore, if the plasmon generator is formed of a corrosion-prone material, the plasmon generator may be corroded. For example, if the plasmon generator is formed of Ag, Cu, or Al, the plasmon generator may be corroded because Ag, Cu, and Al are metals that are relatively easily oxidizable, i.e., prone to corrosion.
Next, a second problem will be described. The second problem relates to excitation and propagation of surface plasmons and to the diameter of the heated spot. To allow the plasmon generator to excite a large number of surface plasmons and to propagate the excited surface plasmons efficiently, it is preferable that the material forming the plasmon generator be high in electrical conductivity. However, if the plasmon generator is formed of a single material having a high electrical conductivity, there arises a problem that the near-field light generated from the near-field light generating part is excessively high in intensity to cause the heated spot to be large in diameter. On the other hand, if the plasmon generator is formed of a single material having a low electrical conductivity, there arises a problem that the plasmon generator cannot excite a sufficient number of surface plasmons and the excited surface plasmons are significantly attenuated before they reach the near-field light generating part.
Next, a third problem will be described. The third problem relates to mechanical strength. More specifically, since the plasmon generator has the front end face located in the medium facing surface, it easily suffers mechanical damage such as deformation if its mechanical strength is low. For example, if the plasmon generator is formed of Ag, Au, Al, or Cu, the plasmon generator easily suffers mechanical damage because Ag, Au, Al, and Cu are relatively soft metals.